This paper presents a novel ecohydrological model in which both the fluxes of water and the relative concentration in stable isotopes (²H and ¹⁸O) can be simulated. Spatial heterogeneity, lateral transfers and plant-driven water use are incorporated. A thorough evaluation shows encouraging results using a wide range of in situ measurements from a Scottish catchment. The same modelling principles are then used to simulate how (and where) precipitation ages as water transits in the catchment.